A coater type conventionally used in paper-making is an applicator apparatus called a jet applicator. This apparatus is a variant of nozzle applicators, wherein the coating mix is applied in a noncontacting manner to the web surface via a very narrow slot orifice. One of the advantages of jet-application techniques is the small pumping rate (3-4 liters of coating mix per second and linear meter of slot orifice) and the small amount of excess coat return flow. The method is particularly suited for web speeds slower than 1000 m/min, because at high web speeds the air film travelling on the web surface begins to disturb the stability of the impinging jet. As the apparatus is rather sensitive to air entrained in the coating mix, an air separator is required in the coating mix circulation, because otherwise the air bubbles of the applied coating mix could cause uncoated spots. The narrow and low-impact discharge from the slot orifice of the jet applicator apparatus does not stress the web being coated and achieves some degree of coat penetration into the web being coated.
However, jet applicators are hampered by being highly sensitive to plugging of the slot orifice, which can be traced to the narrow opening of the orifice. Hence, even very small impurities or hardened coating paste aggregates can get trapped in the slot orifice causing coat defects and requiring cleaning of the orifice. Obviously, a production shut-down is necessary for opening and carefully cleaning the nozzle chamber. To avoid coating mix aggregates from reaching the applicator apparatus and therefrom the web, the coating mix circulation is in most coater installations equipped with strainers designed to remove aggregates and lumps from the circulating coating mix. The strainers are placed between the coating mix tank and the coater unit in the coat circulation. While the screening capacity in the circulation and the separation efficiency of strainers conventionally used in the circulation are sufficient for a majority of coating methods, these screening techniques may pass coating mix aggregates which in jet applicator apparatuses can plug the slot orifice. A source of such aggregates is the coating mix infeed piping section between the strainer and the coater unit. Some amount of the circulating coat easily adheres to this part of the piping or hardens therein so as to become later dislodged as lumps or strips which travel in the coating mix circulation and plug the slot orifice of the jet applicator if allowed to reach that far. Such hardening of the coating mix is chiefly due to small inflow rate to the jet assembly, whereby also the volume change rate and flow velocity of the coat in the piping remain small. While other types of coaters not using the jet-application technique are relatively insensitive to small amounts of coating mix aggregates, the jet applicator has been found to require an almost zero content of aggregates in the coating mix because of the narrow nozzle slot.